An automatic transmission employed in a vehicle is automatically shifted into a desired gear through hydraulic pressure controls by regulating a plurality of solenoid valves in response to various conditions such as a vehicle driving speed, a throttle valve position, and the like. The solenoid valves control various friction elements using hydraulic pressure.
If a driver moves a select lever to a desired shift range, port change of a manual valve occurs, and then various operating elements of a shift mechanism are selectively operated by hydraulic pressure that is supplied from an oil pump according to duty ratio control of a solenoid valve.
Such an automatic transmission includes a plurality of friction elements. When the automatic transmission is shifted into a target gear, some of the friction elements (off-going friction elements) are released from an engaged state, and others (on-coming friction elements) are engaged from a released state. Shift quality of the automatic transmission is mainly determined by release-start timing and engagement-start timing of the friction elements. Various research for improving the shift quality have been undertaken.
A conventional four forward gear automatic transmission includes only one shift portion (primary shift portion), and it is sufficient to control the primary shift portion for shifting. In a conventional five forward gear automatic transmission having two axes that include two shift portions (primary shift portion and secondary shift portion), five forward gears may be realized through independent control of the primary shift portion and the secondary shift portion.
However, in such a conventional control method for the five forward gear automatic transmission, simultaneous control of the primary shift portion and the secondary shift portion is impossible. It is therefore impossible to realize six forward gears in the conventional five forward gear automatic transmission.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known to a person skilled in the art.